The single crystal used as a substrate for a semiconductor device may be, for example, a silicon single crystal, and has been mainly produced according to the Czochralski method (which is abbreviated as CZ method hereinafter).
In the case that a single crystal is produced according to the CZ method, it is produced using an apparatus 10 for producing a single crystal as shown in FIG. 19. The apparatus 10 for producing a single crystal has a part for containing and melting a raw material polycrystal such as silicon, a heat insulating part for intercepting heat, or the like which are installed in a main chamber 11. A pulling chamber 12 extending upwards is connected to the ceiling part of the main chamber 11, and a mechanism for pulling a single crystal 13 with a wire 14 (not shown) is provided at the upper part thereof.
A quartz crucible 16 which contains a melt 15 of a raw material and a graphite crucible 17 which supports the quartz crucible 16 are provided in the main chamber 11, and these crucibles 16 and 17 are supported with a shaft 18 so that it may be rotated and may move up and down freely by a driving mechanism (not shown). To compensate for decline in melt level of a raw material melt 15 as a result of pulling of a single crystal 13, the driving mechanism for the crucibles 16 and 17 is designed to raise the crucibles 16 and 17 as much as the melt level declines.
A graphite heater 19 for melting the raw material is provided so that it may surround the crucibles 16 and 17. A heat insulation component 20 is provided so that the graphite heater 19 may be surrounded by it in order to prevent that the heat from the graphite heater 19 is directly radiated on the main chamber 11.
Moreover, there are provided a cooling cylinder 23 which cools the pulled up single crystal and a graphite cylinder 24 below it. A cooling gas is allowed to flow downward from the upper part of it so that it may cool the pulled single crystal. Furthermore, an inner heat insulation cylinder 25 is provided inside of the lower end of the graphite cylinder 24 so that it may face to the raw material melt 15 to intercept the radiation from the melt surface, and to release the radiant heat from the crystal upward. Furthermore, an outer heat insulating material 26 is provided outside of the lower end of the graphite cylinder 24 so that it may face to the raw material melt 15 to intercept the radiation from the melt surface, and to keep the temperature of the surface of the raw material melt.
The graphite heater 19 which has been usually used is shown in FIG. 20. The graphite heater has a cylinder form, and is mainly made of isotropic graphite. In the direct-current type which has been mainly adopted at the present, the two terminal parts 27 are provided and it has a structure wherein the graphite heater 19 is supported by the terminal parts 27. Two kinds of slits 29 and 30, the upper slit 29 prolonged downward from the upper end of the heat generating part 28 and the lower slit 30 prolonged upwards from the lower end of the heat generating part 28, are provided at several to dozens places so that the heat generating part 28 of the graphite heater 19 can generate heat more efficiently. Such a graphite heater 19 mainly generates heat especially from each heat generating slit part 31 which is between the lower end of the upper slit 29, and the upper end of the lower slit 30 among the heat generating part 28.
A raw material lump is put in the quartz crucible 16 arranged in the apparatus for producing a single crystal as described above and shown in FIG. 19, the crucible 16 is heated by the above-mentioned graphite heater 19, and the raw material lump in the quartz crucible 16 is molten. The single crystal 13 which has a desired diameter and a desired quality can be grown downward of the seed crystal 22 by bringing the seed crystal 22 fixed by a seed holder 21 which is connected to the lower end of a wire 14 into contact with the raw material melt 15 obtained by melting the raw material lump as described above, and subsequently pulling up the seed crystal 22 with rotating it. At this time, so-called necking which makes a diameter thin to be about 3 mm to form a necking part is performed after bringing the seed crystal 22 into contact with the raw material melt 15, and subsequently the diameter is increased to a desired diameter, and thereby a dislocation free crystal is pulled up.
The single crystal produced by such CZ method, for example, a silicon single crystal, is mainly used for fabrication of a semiconductor device. In recent years, in the semiconductor device, integration has become higher, and an element has become finer. The problem of the Grown-in crystal defect introduced while a crystal is grown has become important as an element has become finer.
Here, a Grown-in crystal defect will be explained.
In the silicon single crystal, when a rate of crystal growth is comparatively high, the Grown-in defects such as FPD (Flow Pattern Defect) which is considered to be caused by void wherein vacancy type point defects are gathered exist at a high density all over the region in a radial direction of a crystal, and the region where these defects exist is called V (Vacancy) region. Moreover, when a growing rate is lowered, OSF (Oxidation Induced Stacking Fault) region is generated in the shape of a ring from the circumference of a crystal as a growing rate is lowered. And there exist at a low density outside of this ring defects such as LEP (Large Etch Pit) which is considered to be caused by dislocation-loop wherein interstitial silicons are gathered, and the region where these defects exist is called I (Interstitial) region. Furthermore, if a growing rate is made low, an OSF ring will contract and disappear at the center of a wafer, and the whole surface will become I region.
In recent years, there has been found existence of a region wherein there exists neither defects due to vacancies such as FPD nor defects due to interstitial silicon such as LEP, located between the V region and the I region and outside the OSF ring. The region is called N (neutral) region. Furthermore, it has also been found that there is a region where the defects detected by Cu deposition treatment exist outside of the OSF region and in a part of the N region.
The introduced amount of these grown-in defects are considered to be determined by V/G which is the ratio of a pulling rate (V) and a temperature gradient (G) near the solid-liquid interface of a single crystal (for example, see V. V. Voronkov, Journal of Crystal Growth, 59 (1982), pp 625 to 643). That is, if the puling rate and the temperature gradient are controlled so that V/G may be constant, there can be pulled a single crystal in a desired defect region or a desired defect free region. However, when pulling a single crystal with controlling a pulling rate in a predetermined defect-free region such as N region, it is necessary for the single crystal to be grown at a low-rate. Thus, productivity is significantly lowered and the production cost is increased inevitably. Therefore, it has been desired that the productivity is raised by growing the single crystal at higher rate in order to lower the production cost of the single crystal. It can be theoretically achieved by enlarging the temperature gradient (G) near the solid-liquid interface of a single crystal.
Conventionally, there has been proposed that a single crystal is cooled during being pulled up and a temperature gradient (G) near the solid-liquid interface of a single crystal is made large, by using a chamber and a hot zone structure equipped with an effective cooling means and by intercepting the radiant heat from a heater efficiently, to achieve growing at high rate (for example, International patent publication No. 97/21853). These are performed mainly by changing the structure in a furnace at higher position than a surface of the raw material melt contained in crucible.
Moreover, there has also been proposed a method in which a heat-conductive radiation component is arranged in the lower part of a graphite crucible, the radiant heat is received from a graphite heater, heat is conducted according to heat conduction, and the radiant heat is emitted toward a crucible. By the method, the power dissipation of the graphite heater which surrounds a graphite crucible is lowered efficiently, and the radiant heat to a silicon single crystal during being pulled up is reduced by lowering total quantity of heat, and thereby the temperature gradient (G) at near the solid-liquid interface is made large, to achieve growing at high rate (for example, see Japanese Patent Application Laid-open (Kokai) No. 12-53486).
However, it is hard to say that high-rate growth of a single crystal was fully attained only by these methods, and there has been still room for improvement.